Concrete Technology

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Concrete Technology Finishing Concrete Joints in
Concrete Lecture 20
Eng Eyad Haddad
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Finishing Concrete
WHAT IS FINISHING Finishing is screeding,
floating or trowelling the concrete surface to
densify and further compact the surface of
concrete, as well as giving it the look you
want. Finishing takes place in two stages
1. INITIAL
finishing.
2. FINAL finishing.
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Finishing Concrete

• 1. INITIAL FINISHING
• Concrete is first screeded to the level of the
  formwork, then bullfloated and left to set.
• In some cases screeding leaves a good enough
  finish, especially if floor coverings are to be
  used over the concrete.
• Water then appears on the surface of the
  concrete.This water is called bleed water.

• No final finishing can begin until the bleed
  water has dried up. Mixing
• bleed water with the surface paste will weaken
  it, possibly resulting in a
• dusty surface.

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Finishing Concrete

• Excess bleed water can be removed by dragging an
  ordinary garden hose across the surface of the
  concrete.
• Never try to dry up the bleed water using stone
  dust or cement as this will weaken the concrete
  surface in the long run.
• Once the bleed water dries up and concrete can
  support a persons weight, with only a slight
  marking to the surface, the final finishing can
  begin.

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Finishing Concrete
FLOATING There may be two stages in floating
1. The BULLFLOAT, which is part of the initial
float. 2. The POWER or HAND FLOAT which is
part of the final float. Floating helps compact
and level the surface and close minor
cracks. Floating can be done by hand or with a
power float. Power floating leaves a better
finish than hand floating.
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Finishing Concrete

• 2. FINAL FINISHING
• This involves floating, trowelling, edging,
  jointing or
• patterning the concrete.
• Special finishes such as brooming, colouring or
  patterned
• finishes can be applied to the surface.
• Trowelling leaves a dense, hard, smooth
  adurable surface. The surface should be trowelled
  TWICE. A well trowelled surface will be very
  smooth and can be slippery when wet.
• Trowelling can be done by hand or power trowel.

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Finishing Concrete

• Edging and Grooving
• All the edges of a slab should be finished with
  a special edging tool.
• This gives a neater and stronger edge, less
  prone to chipping.Joints should be planned before
  placing and are usually formed into the concrete
  during finishing.
• Once any surface has been finished it MUST be
  cured.

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Joints in Concrete

• WHAT ARE JOINTS?
• Joints are PLANNED BREAKS in concrete which allow
  it to move and prevent random cracking.
• WHEN TO MAKE JOINTS?
• Joints can be made at two different times
• 1. BEFORE any concrete is poured. As for
  Construction joints or Isolation
• joints.
• 2. AFTER concrete has been placed and compacted,
  as for Control joints.
• Joints are used to control CRACKING in
  concrete.
• Random cracking can weaken the concrete and
  spoil itappearance.

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Joints in Concrete

• TYPES OF JOINTS
• 1. Control Joints
• Wet formed joints are inserted with the use of
  a grooving tool
• to create a plane of weakness which
  conceals where the
• shrinkage crack will occur.
• To be effective the joint must be tooled to a
  minimum depth of
• 1/4 to 1/3 the depth of the concrete, eg
  for 100 mm thick
• concrete joint depth should be a minimum
  of 25 mm to 35 mm.

• Control joints may be made while concrete is
  hardening by slicing it with a thin piece of
  metal.
• The edges of the joints should be finished with a
  grooving or edging tool.
• Alternatively a crack inducer may be cast, or
  pressed into, the concrete.
• Control joints may also be sawn, but timing is
  very important.

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Joints in Concrete

• The edges of the joints should be finished with a
  grooving or edging tool.
• Alternatively a crack inducer may be cast, or
  pressed into, the concrete.
• Control joints may also be sawn, but timing is
  very important.
• Too early and the sawcut can ravel and too late
  the concrete will have
• already cracked randomly.
• A joint may be filled with a flexible filler to
  minimize water entry and to
• prevent stones etc entering which may later
  cause spalling of the concrete.

• The position and number of control joints must be
  carefully planned.
• Control joints in an unreinforced concrete
  slab should divide it into
• roughly square areas. (ie A one-metre-wide
  path needs a control joint
• about every one metre).

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Joints in Concrete

• Control joints in unreinforced concrete should be
  located at a spacing of a width to depth ratio of
  about 20 (to 25) to 1 ie a 100 mm deep slab
• should have joints every 2 to 2.5 m.
• In steel reinforced slabs the joint spacing is
  controlled by the area of steel.
• The more steel there is, the further apart the
  joints can be.

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Joints in Concrete

• 2. Isolation Joints
• An isolation joint totally separates a concrete
  element from
• another concrete element, or a fixed object
  such as a wall or
• column, so that each can move and not
  affect the other.
• The joint filling should be full depth and soft.
  It can be made of cork, foam rubber, or some
  other flexible material.

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Joints in Concrete

• 3. Construction Joints
• A construction joint is a place where work
  finishes temporarily.
• Formwork is used to support the edge of the
  concrete already in place so that it doesnt just
  collapse.
• Concrete is finished square and the
  reinforcement normally runs through the joint.

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Joints in Concrete

• When placing begins again
• Remove the formwork and brush any loose material
  from the old surface.
• Roughen the old surface, to expose the coarse
  aggregate, to help the new concrete bond
  properly.
• Pour the new concrete against the old surface.

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Joints in Concrete
PLANS The position of ALL JOINTS should be shown
on the plans for any concrete slab.
THE END